1. Field of the Invention
The present invention relates to a focal length display device of a camera. More particularly, the invention relates to a focal length display device of a camera which comprises a zoom lens which is continuously varifocal due to a variable power function of a group of lenses arranged on a same optical axis and to which camera a plurality of converters such as a tele-converter and a wide-converter are attachable wherein the focal length of each of the converters can be visually confirmed.
2. Description of the Related Art
There have been used a single focus camera and a double focus camera comprising a telephotolens and a wide angle lens selectably provided thereto. With respect to such a single focus camera and the double focus camera, the focal distance at each shot is not the matter, since the user of the camera only need to know the single focal length of the camera or those of the telephotolens and the wide angle lens, respectively, of the double focus camera.
Also, when the tele-converter or the wide-converter is attached to the single focus camera or the double focus camera, the focal length of the camera can be easily calculated by multiplying the focul length of the camera by the magnification of the tele-converter or the wide-converter. Therefore, it is unnecessary to display the focal length of the camera at the time of taking an object.
For the reason mentioned above, the single focus camera or the double focus camera does not comprise a means for displaying the focal length thereof.
On the other hand, with respect to the camera which uses a variable power lens such as a zoom lens or a varifocal lens as an objective, the camera comprises a focal length display device which is constructed for example in such a way that an operator (user) can confirm the focal length by moving a reference mark (index) pressed on a barrel of the variable power lens and reading the scale of the focal length indicated by the index.
However, further convenient focal length display devices are being required since the zoom lenses are widely used nowadays which enables to take photos at a desired focal length within the range of the zoom lens.
Japanese Utility Model Application Laying Open (KOKAI) No. 63-10713 discloses a focal length display device which mechanically displays the focal length of the camera.
Also, Japanese Utility Model Application Laying Open (KOKAI) No. 63-174337 discloses a focal length display device which electrically displays the focal length of the camera. This electric display device comprises an objective system which has a zoom function and is mounted on a body and a zoom display unit which is provided on the body and arranged in such a way that the zooming characteristic factors such as focal length, field angle and magnification of the zoom lens are converted to electric signals and displayed numerically in a liquid crystal display.
The zoom lens camera provided with the electric focal length display unit mentioned above is very convenient to use since the focal length is displayed in the liquid crystal display according as the zoom lens barrel is rotated and index position thereof is shifted.
To enlarge the variable range of the zooming function, there are some cameras that comprise a zoom lens to which a tele-converter or a wide-converter can be attached. However, such a camera is not constructed so that the focal length is displayed at the time when the tele-converter or the wide-converter is attached to the zoom lens.
The tele-converter has a magnification of for example, 1.4, 1.5 or 2. Also, the wide-converter has a magnification of for example, 0.7 or 0.5. When a tele-converter having a magnification of a whole number times for example 2 times is attached, the user can easily calculate the focal length of the zoom lens combined with the tele-converter simply by multiplying the displayed numeral by 2. However, it is not that easy with respect to a converter having a magnification of a decimal fraction number, for example a tele-converter having a magnification of 1.4 or a wide-converter having a magnification of 0.7.
Especially, with respect to the zoom lens, the focal length thereof is continuously variable, which makes it difficult to know the focal length of the zoom lens combined with the converter at the time when the magnification of the zoom lens is changed from the display of the focal length of the zoom lens.